There is a great need for gas turbine engines and auxiliary power units providing improved performance, lower cost, better maintainability, and higher reliability. The Integrated High Performance Turbine Engine Technology program has provided significant advances in compressors, turbines, combustors, materials, generators, and other technologies. In order to make significant improvement in power vs. weight ratio, gas turbine engines and auxiliary power units must operate at higher speed and at higher temperature. In addition, the complicated oil lubrication system must be eliminated to facilitate higher temperature operation, and to reduce weight and cost. Magnetic bearings have shown great promise to meet goals of the Integrated High Performance Turbine Engine Technology program. However, in many applications, use of magnetic bearings is limited due to requirements of auxiliary bearings, cooling methods, weight and cost.
Foil air bearings do provide a promising alternative to magnetic bearings. Foil air bearings are successfully being used in air cycle machines of aircraft environmental control systems. Today, every new aircraft environmental control system, either military or civilian, invariably makes use of foil air bearings. Older aircraft are being converted from ball bearings to foil air bearings. Certain military aircraft air cycle machines used ball bearings up to 1982 and since then, are using foil air bearings. The reliability of foil air bearings in air cycle machines of commercial aircraft has been shown to be ten times that of previously used ball bearings in air cycle machines.
In spite of tremendous success of foil air bearings for air cycle machines, their use for gas turbine engines has been limited. This is due to the fact that gas turbine engines operate at higher temperatures and exhibit higher radial and axial loads. The radial loads are carried by foil journal bearings such as shown in U.S. Pat. No. 3,382,014 and discussed in ASME paper 97-GT-347 (June 1997) by Giri L. Agrawal entitled “Foil Air/Gas Bearing Technology—An Overview.” The axial loads are carried by foil thrust bearings such as shown in U.S. Pat. Nos. 3,382,014 and 4,462,700. In recent years, the load capacity of foil journal bearings has increased to a level which is satisfactory to carry radial loads of a typical gas turbine engine. However, the thrust load capacity requirement of a foil thrust bearing to be used for a gas turbine engine could be as much as four times that supplied by present day thrust bearing technology.
One solution to achieve higher thrust load capacity for a foil thrust bearing in a gas turbine engine is to increase the diameter of the thrust bearing. But larger diameters require greater radial space, increase stresses in the thrust runners, and increase power loss. Load capacity of a foil thrust bearing is also dependent on the flatness of the bearing. As flatness is maximized, load capacity increases. Due to various manufacturing tolerances and constraints, and also due to various operating conditions, keeping the thrust bearing very flat is a difficult task. The problem becomes more difficult as the size, and especially the diameter, of the thrust bearing increases.
The use of foil bearings in turbomachinery has several advantages:
Higher Reliability—Foil bearing machines are more reliable because there are fewer parts necessary to support the rotative assembly and there is no lubrication needed to feed the system. When the machine is in operation, the air/gas film between the bearing and the shaft protects the bearing foils from wear. The bearing surface is in contact with the shaft only when the machine starts and stops. During this time, a polymer coating, such as Teflon®, on the foils limits the wear.
Oil Free Operation—There is no contamination of the bearings from oil. The working fluid in the bearing is the system process gas which could be air or any other gas.
No Scheduled Maintenance—Since there is no oil lubrication system in machines that use foil bearings, there is never a need to check and replace the lubricant. This results in lower operating costs.
Environmental and System Durability—Foil bearings can handle severe environmental conditions such as shock and vibration loading. Any liquid from the system can easily be handled.
High Speed Operation—Compressor and turbine rotors have better aerodynamic efficiency at higher speeds, for example, 60,000 rpm or more. Foil bearings allow these machines to operate at the higher speeds without any of the limitations encountered with ball bearings. In fact, due to the aerodynamic action, they have a higher load capacity as the speed increases.
Low and High Temperature Capabilities—Many oil lubricants cannot operate at very high temperatures without breaking down. At low temperature, oil lubricants can become too viscous to operate effectively. As mentioned above, foil bearings permit oil free operation. Moreover, foil bearings operate efficiently at severely high temperatures, as well as at cryogenic temperatures.